Method of detecting defects in ingition shielding



July 19, 1938. M. F. PETERS 2,123,910

METHOD OF DETECTING DEFECTS IN IGNITION SHIELDING Filed Oct. 12, 1937 g- 2 v INVENTOR MELV/LLE F. PETERS ATTORNEY Patented July 19,. 1938 PATENT OFFICE IGNITION SHIELDING Melville F. Peters, Beltsville, Md.

Application October 12 3 Claims.

, 1937, Serial No. 168,593

(Granted under the act of March 3, 1883, as

amended April 30,

My invention relates to a method of detecting defects in electrical shielding for ignition circuits and more particularly to a potential method wherein the ignition current flowing 5 through the shielding is utilized for producing a potential drop that is indicative of a defect.

The suppression of electromagnetic and electrostatic radiation from ignition systems by on closing the conductors in metallic casings soas to eliminate interference with radio reception and transmission is a well-known expedient. If such shielding is to be effective it is essential that the ignition circuit be encased by a metallic surface; that the enveloping metallic surface be grounded .at its ends and at suitable intervals therebetween; and that the enveloping metallic surface be electrically continuous throughout its length. Failure of the enveloping metallic surface to properly and effectively shield against the radiation generated by the ignition circuits is caused principally by lack of electrical continuity along the enveloping metallic surface or an increase in resistance or impedance of the enveloping metallic surface over that at which optimum shielding is obtained. This lack of electrical continuity or increase of resistance or impedance generally takes place at the connectors or joints forming a part of the enveloping metallic surface- As long as the contacting surfaces of the 'connectors or joints are in intimate metallic contact and remain free from dirt, oil, gasoline, or

other extraneous matter, the enveloping metallic surface will efiectively shield. If,'h'owever, thecontacting surfaces of the connectors or joints are not in intimate metallic contact or do not remain free from extraneous matter, there will be a lack of electrical continuity or an increase of resistance or impedance through the connectors or joints with attendant interruption or 40 malfunctioning in the shielding properties of the enveloping metallic surface.

It has been determined experimentally that the radiation from ignition circuits is suppressed because the major portion of the ignition current returns through the shielding surrounding the conductors; that this is due to the formation of a low impedance return circuit by an eificient shielding because of its combination with and proximity to the high tension conductors; and that this low impedance return circuit with the current flowing therethrough results in a relatively low potential drop along, the shielding.

.If, on the other hand, a defect occurs in the shielding tending to increase its resistance or impedance and which may be due to poor installation or deterioration occasioned by use, less current returns through the shielding and more through the internal combustion engine to which the shielding is grounded. Since the electromagnetic field associated with the return current underthese conditions is no longer sufii cient to neutralize the electromagnetic field set up by the current flowing to the spark plug through the' high tension ignition cable, radia-. tion from the ignition circuit will ensue. Furthermore, it has been determined empirically that under these circumstances a relatively high potential drop occurs across the shielding defect; and that this drop is considerably in excess of that obtaining along the shielding when it is effective to suppress radiation. Thus, there is provided a convenient method of detecting defects in shielding for ignition circuits which com-v prises measuring by means of any suitable instrumentality the potential drop along the shielding occasioned by the flow of the return ignition current. If this drop is in excess of that known to be characteristic for eifective shielding it is known that the shielding in question is defective in that it fails to suppress the electromagnetic and electrostatic radiation engendered by the ignition system.

In the light of the foregoing, it is the prin cipal object of-my invention to provide a potential method for detecting defects in electrical shielding for ignition circuits wherein the return ignition current through the shielding is utilized and which is at once simple, convenient and, effective. Y

Other objects and many of the attendant ad-' vantages of this invention will be'readily appreciated as the same becomes better understood by reference to the following description when considered in. connection with the accompanying single sheet of drawings wherein:

Fig. 1 shows schematically a conventional prior art, shielded ignition system and a potential measuring apparatus by which the method of the present invention is carried into effect; and,

Fig. 2 is a diagrammatic representation of a conventional prior art capacity-resistance coupled amplifier circuit which may be used in measuring potential drops along the shielding assembly.

Turning now to Fig. 1 of the drawing there is shown depicted therein any suitable spark generator I which is connected by means of the high tension ignition cable 2 to the spark plug 3. For shielding the high tension cable 2 there is provided the electrically conductive conduits l detachably connected by the joint or connector 5 and surrounding the parts of the ignition systern: in the manner shown. The ends of the shielding assembly are shown as grounded to the internal combustion engine, a fragmentary portion of which is indicated by the reference character 6.

For measuring the potential drop along the shielding assembly a suitable probe 1' is employed, one end of which is designed to make intimate metallic contact with any desired point of the shielding assembly and the other of which is connected to any convenient potential measuring device 8.

Any device known to the prior art that is capable of measuring the magnitudes of voltage here encountered may be advantageously employed. As illustrating one such device there is shown depicted in Fig. 2 a conventional capacityresistance 'coupled amplifier employing three stages identified by reference characters 9, l0

and I I and wherein the vacuum tubes are of the screen grid type. Although three stages of amplification are shown this number is not deemed critical and may be varied to suit the conditions at hand. In the circuit depicted in the drawing, the meter [2 is provided with two scales, one of which serves for measuring the filament voltage applied to the tubes [3 and I4 and the other of which is adapted for measuring the plate voltage. The filament and plate voltages are adjusted by means of the rheostats l5 and I6, respectively. Since the tube I! may differ from tubes l3 and I4, a separate meter I8 is employed for measuring its filament voltage. The meter I9 indicates the magnitude of the voltage applied to the probe 1. The battery 20 supplies filament current for all three tubes. Battery 2|, on the other hand, supplies the plate and screen grid voltages for tubes l3 and I4 and the plate, screen and suppressor grid voltages for tube IT. The probe 1 of Fig. 1 is indicated diagrammatically by the same reference character in Fig. 2 of the drawing. No further description of the circuit in Fig. is deemed necessary since it is without novelty and of a well-known, conventional prior art type.

In carrying out the method of my invention let it first be assumed that the shielding assembly comprising the conduits 4 and joint 5 is without any defect and that it is effective in suppressing all electromagnetic and electrostatic radiation generated by the current flowing through the ignition cable 2. Under these conditions, if the potential drop is measured along the shielding assembly by grounding one end of the measuring device 8 and placing the metallic end of the probe 1 into contact with the said assembly, the measurable voltage drop, if any, will be of a very small magnitude. If now, as a resultof use, oil or any other foreign substance comes in contact with the exposed portions of the joint 5 and thereby increases its resistance with attendant impairment in the shielding ability of the assembly, a voltage drop of relatively large magnitude will occur across the joint 5 thus advising the operator that the shielding is defective at this point. Thus, there is provided a method of detecting defects in electrical shielding which is at once simple, effective and convenient in use.

According to the provisions of the patent statutes I have set forth the principle and mode of operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically illustrated and described.

The invention herein described and claimed may be used and/or manufactured by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. A method of detecting defects in electrical shielding for ignition circuits which comprises the steps of passing ignition current through the shielding and measuring the potential drop there"v along.

2. A method of detecting defects in electrical shielding for ignition circuits which comprises the steps of passing ignition current through the shielding and measuring the potential drop between selected points therealong.

3. A method of detecting defects in a joint of an electrical shielding assembly for ignition circuits which comprises the steps of passing ignition current through the shielding assembly and measuring the potential drop across the joint.

MELVILLE F. PETERS. 

